I spot Victoria Wood across the room before she sees me. I'm late, and she's waiting on a sofa in her private members' club – but her demeanour is so surprising that instead of rushing over, I pause for a second and watch. She reminds me of women travelling alone late at night on public transport, with that mouse-like air of self-containment and suspicion, as if half expecting someone to try and steal her handbag. Surrounded by tables of noisy Soho media types, Wood gazes into her lap, and when a waiter clears away her breakfast she shrinks even further into herself, jumpy with self-effacement. There she sits, the most celebrated person in the room by a million miles, looking extraordinarily vulnerable.

Half an hour later, the same woman is relating her indignation at BBC executives who try to tell her how to do her job. "And you think, well that's fine, but what's your qualification for telling me what's funny? Please don't tell me what's funny, cos I know what's funny. And you probably don't. That's why I'm on television and you're not."

To describe Wood as a paradox might be almost as much of a cliche as to call her a national treasure. In an era when convention has it that celebrity requires beauty, youth and edge, the actor and comedian remains one of the country's most widely loved stars, once even beating the Queen Mother into second place to win a poll of People You'd Most Like To Live Next Door To. Wood carries herself so meekly, though, you wonder if anyone has told her. And yet, ever since she can remember – and long before she had any evidence for it – her faith in her talent has been impregnable.

Wood's talents are truly prodigious, as both a performer and a writer. A household name ever since dominating the sketch show format in the 80s, her standup tours have sold out the Albert Hall for 15 nights running, not once but twice, and she wrote and starred in the hit sitcom Dinner Ladies, and the Bafta-winning drama Housewife 49. She has written a musical, presented documentaries about the British Empire and the diet industry, and has recently made another for the charity MAG (Mines Advisory Group) about clearing unexploded ordnance in Laos. It's narrated with a foreign correspondent's effortless authority that would give Kate Adie a run for her money.

Yet when we meet, she is still smarting, nine months later, from having had her 2009 special, Victoria Wood's Midlife Christmas, demoted from its promised prime-time Christmas Day slot to the night before. The BBC executive who made the decision didn't even, she says, bother to tell her.

"I've still not met the person in question, she's not ever introduced herself to me, she's walked past me in the corridor without speaking to me. I just think it's rude. I treat them with manners, I turn up on time and I make my shows on time and I make them as well as I can. And if somebody's got a problem with something I'm doing I think they should step into the same room or email me or telephone me, and not send a winged monkey to talk to me about it."

Victoria Wood in Dinner Ladies. Photograph: BBC

Does she think it was symptomatic of wider developments at the BBC, or just an unfortunate one-off? "No, I think a lot of us feel that way. That there are great hierarchies and you can't have a personal relationship. So it's just defeating because it becomes faceless. There was never a golden age when they were hugging and kissing you, but they used to leave you alone. Now it's so prescriptive and everyone's chucking in their two penneth – and I want to be reasonable, and I want to be cooperative. But I want there to be mutual respect, and there isn't any."

As she talks she looks down a lot, twisting her hands between crossed legs, but she is determined to make her point. "I just find the layers of people you have to deal with tiresome. And you think, 'Well, fine, you make it then, I won't make it at all. I'll go home and put the washing on, fine.' You used to be trusted and now I feel like I'm not trusted, and I don't like it. Not valued, not needed on voyage – that's what it makes you feel like. I'm not trying to pull a big huff. It's not a celebrity huff. It's a working person's huff, and I think it's a justified huff, and it's on behalf of all of us who feel miffed and sidelined and overly interfered with."

Wood is currently casting for a crew to work on a BBC biopic she's producing about Morecambe and Wise, but has to recruit locally to avoid paying travel and accommodation expenses. It all sounds more like provincial am-dram than a flagship production, so I wonder how she feels about the seven-figure sums paid to the corporation's star presenters.

"Well, I know what I get for certain jobs, and when I see what other people get it seems like an awful lot," she laughs. "It's a very bizarre area because you're worth what someone will pay. But when it's coming out of the licence fee, some of the pay seems disproportionate. Naming no names, of course."

Jonathan Ross, I suggest, would have been an obvious one. "Yes, wouldn't it," she says drily. Does she think, "Hold on a minute, why don't I get paid anything like that?"

"No, I think some of that money could have paid for actors, wigs, costumes, orchestras. Things that would have made a broadcast channel much more rich and varied instead of lumping it all to one face. I'm not envious of people's money, but if I'm told I can't have a wig, I can't have a costume – well, when I see big sums, to me that's wig money, it's the score for your drama, it's all the things that make television so brilliant." She looks down into her lap again and finishes quietly, "That's all I think."

An Audience with Victoria Wood. Photograph: Rex Features

Wood is a fascinating mixture of humility and steel. She could be the lovechild of Alan Bennett and Pam Ayres, and like a lot of quiet people has a trace of passive aggression, landing sly blows before retreating back into her shell. When I ask if it would be obvious to an observer at a dinner party that she was a professional comic, she says, "No, if I'm in a group I won't dominate. I never speak for the sake of speaking." She adds, almost under her breath, "I get annoyed if people do – as they tend to do. I think, 'Oh, shut up'." Then quick as a flash, out comes an apologetic little laugh.

The origins of her insecurity are not opaque. Wood had a pretty miserable childhood, lonely and overweight and awkward, growing up in a ramshackle house in rural Lancashire in the 60s with two thin, gregarious elder sisters. "I felt they were better at everything, and they were very glamorous. They were always out, and I was always in. I had a very low opinion of myself, very low. I was always looking at other people and thinking they had a better life. Partly because I was fat; I was always very envious of thin people particularly. It was all my projection, I had no idea if it was true, but I thought if you were thin you were happy, if you had a boyfriend you'd be happy, if you had a different family and lived in a nice, clean semi, you'd be happy. That was my feeling."

Where her burning ambition for fame came from, though, doesn't seem quite as clear, for by her own account she was the very antithesis of the precociously cocky stage kid. Yet she knew from an early age that she wanted to perform in show business.

"But I think it's not a paradox," she offers. "I think that it's often these children who feel they don't quite fit in, they're not part of the group – I think a lot of very seemingly shy people have got this ability to connect with a group, rather than one-on-one. And I just knew, I just knew I had that. I couldn't define it, but I knew I had it."

At first her self-belief appeared entirely justified. Studying drama at Birmingham University, she won ITV's New Faces talent show in 1973, and became a regular novelty act on That's Life!. "I'd had quite a good start," she agrees. "But then it all tailed off because I didn't know how to move it on. I did everything you could do wrong, really. I was very depressed, it was a very bad stage in my eating and a very bad stage in my isolation. I lived in a bedsit, didn't spend much time with friends, was eating a lot. It was a very grey period. I thought, I've had my chance and I've blown it. I remember thinking very clearly, I'm 23 and I'm too old. So I was lucky later that it worked out. If I'd not met Geoffrey, I'm not sure that it would have."

Wood met Geoffrey Durham, a magician, in 1976, when he played Buffalo Bill in a Wild West Show in Leicester, opposite her Wilhelmina Fifty. It doesn't sound like the most romantic of beginnings, but she credits him with giving her the confidence to keep pursuing her career, and they were together for 26 years, marrying in the 80s and moving from Morecambe to north London with their two children in the 90s. She was stunned when he left her in 2002, and doubts she'll ever recover from the loss and regret. People were quite wrong, she insists, to assume their marriage was destroyed by the discrepancy between the couple's professional success, and she seems very anxious to say nothing that could upset her ex-husband,

"Looking back," she offers cautiously instead, "I think I put myself under a lot of pressure to be a very present mother, and a very big comedian, and to be sort of running the household. So there was a lot to do, and it's difficult to keep everything in balance, and I was always tired, short of sleep, resentful. You could be onstage but you've got to come home and make a costume for a school play – and I think I probably took it all too seriously. I was anxious about everything being OK, and doing everything properly. More than I needed to be. I'm not so much like that now, I think."

Her daughter is now a choral scholar at Cambridge, and her son is about to leave home for Leeds to study music technology. At 57, therefore, Wood is about to find herself living alone. "Yeah, I'll be like Miss Havisham, sat there all by myself," she laughs softly. Is she scared?

"Yes, yes. Well I'm trying to face it. I've thought about it a lot. I suppose I'm scared of bouncing back in my head to living in a bedsit. I won't be living in a bedsit, I'm not who I was then – but I think it has echoes for me of the past if I suddenly find myself on my own. Back to being 21 and eating tinned mince. But actually I'm not that person any more."

Having battled with her weight for most of her life, and suffered periods of depression that she attributes to disordered eating, today Wood is healthy and trim. She credits therapy, and says tentatively, as if fearful of jinxing herself, that her relationship with food is now as close to normal as most women's. When she first became famous, though, she was co-opted into the category of the happy-go-lucky, roly-poly funny woman. "Yes," she agrees, "but I never bought into it. I knew it was a lie. I was just fat, and that was that, and that was the cross I bore." When she would hear others, such as Dawn French, claim to love their curves, I wonder if she believed them?

"No, I thought they were lying. Either to themselves or to the world. I don't think there's any happy fat people in the world, ever. Yeah, sure, you can have a bit of a tum or a big bum and be absolutely fine with it," she adds quickly. "But if you've got an odd relationship with food, that means something else is going on that means you're not comfortable with yourself.

"I never liked food, I've never been happy with food. I like it a bit more now, but I never liked it then. It was always a problem to me. Every bit of food, it was either bad food or good food – you know, everything was a problem, a rule or a breaking of a rule. Other people would go, 'Ooh, I'm really looking forward to my dinner', and I'd think, 'Oh, I'm not, I've been eating all afternoon.' I was never hungry, I ate all the time, so a meal was just something you got through before you got back to what you really wanted to eat, which wouldn't be meat and two veg – how utterly boring, it just had to be got through. I couldn't understand people who wanted to sit down and have dinner and a chat and eat slowly. I just thought no, I want to eat a lot – preferably in private."

In a way, she says, it's quite a relief to be 57, and out of danger of being judged on her appearance. She hates wearing high heels, partly on account of the discomfort, but "also feeling that you can't run. I don't like that feeling that you can't get away." When I ask if she would have had the same ambition for fame if she were young today, in a world of Heat and OK!, she says, "Well, there's always been good-looking people – and I like looking at them as much as anybody, so I'm very happy they're on the telly. But if you look at Peter Kay or Alan Carr, none of them would win Miss World. And the good old British public, they don't seem to mind if someone looks like a baked potato. In fact, maybe they quite like it.

"The red carpet is fine for girls, but I think there should be another red carpet, maybe a blue one, for me and Alan Carr and Peter Kay to walk down in our flat shoes at the back. Then we could get in without inching along being asked about Big Brother." She starts to laugh. "I think that would be a really good idea."